Whether QQ will slow your computer down is depend on what's hardware of your computer. In general, if your CPU is level of Core 2 or above it, don't worry about it. On techinically, QQ won't expose users much more than you didn't have it, the safety is your rule to face the threat of internet. My advice is to install it and enjoy it except if you were someone who always been watched by gfw.

QQ in the past installed all kinds of crap. Nothing I would consider harmful per say, but just lots of crap. It's better now. You could also try Tencent Messenger, or the English/international version of QQ.

The main version of QQ fills your computer with all sorts of junk but the international one (which uses the same protocols so can connect with Chinese users) is safe but only available in English, Japanese and French (no Chinese).

Web QQ is the most safe as it runs QQ in a virtual machine inside your browser (but I believe it only has Chinese GUI).

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